Introduction
Have you ever made homemade ice cream? It can be a lot of fun, and you end up with a tasty frozen treat! A lot of interesting chemistry is actually needed to make ice cream. For example, think about how you start out with refrigerated (or even room-temperature) ingredients and then need to cool them down to turn them turn into ice cream. How do the ingredients change during this process? How important do you think they get cooled to a certain temperature? In this science activity you'll make your own ice cream (in a bag!) and explore the best way to chill the ingredients to make them become a delicious reward!

Background
To make ice cream, the ingredients—typically milk (or half and half), sugar and vanilla extract—need to be cooled down. One way to do this is by using salt. If you live in a cold climate, you may have seen trucks spreading salt and sand on the streets in the wintertime to prevent roads from getting slick after snow or ice. Why is this? The salt lowers the temperature at which water freezes, so with salt ice will melt even when the temperature is below the normal freezing point of water.

Technically, the temperature that the salt lowers is called the freezing point. When a freezing point is lowered, such as by adding salt to water, the process is called freezing-point depression. As we'll see in this activity, freezing-point depression is not unique to solutions made of water and salt; it also happens with other solutions. (A solution is made when a substance, such as salt, is dissolved and becomes a solute. The medium into which it is dissolved is a solvent—typically a liquid, like water.)

Materials

Measuring spoons

Measuring cup

Sugar

Half and half (Alternatively, milk or heavy whipping cream may be used.)

Vanilla extract

Salt (Different types of salts, such as table salt or rock salt, should work but may give slightly different results.)

Two small, sealable bags such as pint-size or sandwich-size Ziplocs

Two gallon-size sealable bags

Eight cups of ice cubes

Oven mitts or a small towel

Timer or clock

Preparation

In each small bag, place one tablespoon of sugar, one half cup of half and half (or milk or heavy whipping cream) and one quarter teaspoon of vanilla extract. Seal up each bag after adding the ingredients. Keep the bags in the refrigerator until you are ready to continue on to the procedure.

Procedure

Add four cups of ice cubes to one of the large, gallon-size bags. Then add one half cup of salt to the bag. What do you think the salt will do?

Put one of the small bags you prepared into the large bag with the ice cubes. Be sure both bags are sealed shut.

Put on oven mitts or wrap the bag in a small towel and then shake the bag for five minutes. Feel the smaller bag every couple of minutes while you shake it and take a peek at it. What happens to the ingredients over time? When five minutes are up, how do the ingredients look? What about the ice cubes—how do they change over time and how do they look by the end?

Now add four cups of ice cubes to the other large, gallon-sized bag, but this time do not add any salt to it. What do you think will happen without using salt?

Put the other small bag you prepared into this large bag. Be sure both bags are sealed.

Put on oven mitts or wrap the bag in a small towel and then shake the bag for five minutes, as you did before. Again, feel the smaller bag every couple of minutes while you shake it, and take a peek at it. What happens to the ingredients over time now? When five minutes are up, how do they look now compared with last time? What about the ice cubes—did they change in the same way?

You can also compare how cold the different ice cube bags feel. Does one feel much colder than the other?

If you successfully made some ice cream, you can enjoy it now as a tasty reward for your chemistry challenge! If the ingredients in one of your bags did not become ice cream, check out the extras below for tips on turning them into ice cream.

Extra: If one of your bags did not make ice cream, try putting it back inside the large bag that had ice cubes and salt and then shake it for five minutes. Did it turn into ice cream? Why do you think you got the results that you did?

Extra: Instead of half and half, you could use milk or heavy whipping cream to make ice cream in this activity. How does making ice cream with half and half compare with using milk or heavy whipping cream?

Extra: You can make ice cream in this activity using different types of salts, but you may get different results. How does ice cream made in a bag with table salt compare with ice cream made with rock salt or some other type of salt? Can you explain your results?

[break]Observations and results
Did the ingredients that were shaken in the large bag with the ice cubes and salt turn into ice cream whereas the ingredients that were shaken with ice cubes alone (and no salt) remain fluid, and not harden into ice cream?

You should have seen that the ice cubes in the large bag with salt melted much more, and felt much colder, than the ice cubes in the large bag without salt. Because it was cold enough (several degrees below freezing), the ice cube bag with salt should have been able to cool the ingredients enough to harden them and turn them into ice cream whereas the ice cube bag without salt wasn't cold enough to do this, leaving the ingredients fluid. (If you put the fluid ingredients back in the bag with ice cubes and salt and shook them for about five minutes, the ingredients should have been cooled sufficiently to turn into ice cream!)

If you have ever made ice cream with an old-fashioned hand-crank machine, you probably packed a mixture of ice and rock salt around the container holding the cream. The ice–salt combination gets colder than pure water ice and can freeze the ingredients in the ice cream machine (and in the bags you used in this activity), turning them into ice cream. (This is the same process that occurs when icy roads have salt spread on them to melt the ice, keeping the roads less slippery at lower temperatures.) Although pure water freezes at 0 degrees Celsius (32 degrees Fahrenheit), water mixed with salt will only freeze at colder temperatures below 0 degrees C.

Cleanup
If you haven’t already, you can enjoy your tasty ice cream treat now or save it in the freezer for later.

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